The subject matter described herein relates generally to wind turbines and, more particularly, to a method and apparatus for generating power in a wind turbine.
Generally, a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a tower.
Some wind turbine configurations include double-fed induction generators (DFIGs, also known as dual-fed asynchronous generators). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid. Alternatively, some wind turbine configurations include, without limitation, alternative types of induction generators, permanent magnet (PM) synchronous generators, electrically-excited synchronous generators, and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
At least some known electric utility grids include one or more series-compensated transmission lines. Such transmission lines often create subsynchronous resonance currents that may be lightly damped. When at least some known wind turbines are electrically coupled to such transmission lines, the wind turbines decrease the damping of the subsynchronous currents. As such, the subsynchronous currents may increase in amplitude and may cause a fault or “trip” to occur and render the wind turbine inoperable. Moreover, such subsynchronous currents may damage or otherwise shorten a lifespan of one or more components of the wind turbine and/or the electric utility grid.